Thirty Years of Data Show Uruguay River Water Quality Improved but Never Excellent

Water quality in the mid-lower Uruguay River has improved over the past three decades, but it has never reached an excellent rating, according to a 30-year analysis of monitoring data published in Ecological Processes.

The researchers assembled a database of 246 observations covering 42 water quality parameters at three main-channel sampling sites spread across 440 kilometers of the river: Monte Caseros–Bella Unión, Fray Bentos–Gualeguaychú, and La Concordia. The parameters spanned physicochemical measurements, nutrients, metals, microbiological indicators, pesticides, and persistent, bioaccumulative and toxic substances. The dataset was not continuous, with gaps in monitoring from 1996 to 1998 and again from 2005 to 2012, the latter coinciding with the diplomatic crisis between Uruguay and Argentina over a pulp mill.

Seventy percent of the parameters that have threshold guideline values in the current river regulations exceeded those limits at least once during the 30-year period. Chlorides and iron were the most frequent offenders, with 45 percent of their measurements surpassing guideline values. Iron concentrations exceeded the guideline only from 2012 onwards, and its frequency of non-compliance rose significantly over time, showing a strong positive correlation with year. Other parameters, including dieldrin, alkalinity, chlorides, cadmium, chromium, copper, lead, zinc, Kjeldahl nitrogen, total phosphorus, and phenols, showed no significant correlation between non-compliance frequency and year.

The clearest improvement involved legacy pesticides. Concentrations of aldrin, DDT, dieldrin, heptachlor, heptachlor epoxide, and lindane all declined significantly over the 30 years, and these compounds, along with Kjeldahl nitrogen and total nitrogen, recorded their highest average values in the 1990s. The researchers attribute the pesticide decline to a coordinated regional ban: Uruguay, Brazil, and Argentina prohibited these persistent organic contaminants, with Uruguayan and Argentine laws taking effect in 2005. The team argues this reaffirms the value of coordinated public policies for managing transboundary water resources.

Metals told a different story. Cadmium, chromium, copper, lead, and mercury showed their highest average concentrations in the 2000s, while iron peaked in the 2010s. The authors suggest heavy metal pollution in the basin may derive from agricultural and industrial activities, urban expansion, and mining, noting that the upper basin hosts extensive amethyst geode extraction that generates metal-laden runoff. Lead concentrations above recommended levels during the 1990s and 2000s match earlier reports from the upper Uruguay River and the Río de la Plata, where lead is attributed to effluents from large cities and industries. Mercury biomagnification has previously been documented in large piscivorous fish such as dorado in the river, with muscle concentrations exceeding FAO Codex Alimentarius recommendations.

Using Bayesian generalized linear models, the team examined how each parameter responded to time and to river discharge, measured at the Paso de los Libres hydrometric station upstream of all sites. The responses varied across sites, with most effects observed at the upstream site, RUY480, above the Salto Grande reservoir. At all three sites, calcium, BOD5, and magnesium decreased over time while iron and nitrite increased. Discharge itself mattered: at RUY480, turbidity, total suspended solids, nitrate, total nitrogen, and total phosphorus increased with discharge, suggesting predominantly diffuse origins from catchment runoff, while chlorides, conductivity, pH, sodium, magnesium, and total hydrocarbons decreased with discharge, pointing to point sources such as industries and cities.

Nutrient dynamics depended on hydrological conditions. Under low-discharge conditions, total phosphorus and nitrogen, nitrates, and nitrites increased with discharge, but under average and high-discharge conditions total nutrients showed no relationship with discharge, a pattern the authors interpret as nutrient response rapidly reaching an asymptote. Nitrite increased with year under all three hydrological conditions. Notably, despite ongoing agricultural intensification in the basin, total nutrients showed no trend over time except for nitrite. The authors hypothesize that anthropogenic pressures may have shifted, with nutrients once dominated by industrial and sewage point sources now increasingly supplied by diffuse agricultural inputs.

When the parameters were combined into the Canadian Council of Ministers of the Environment Water Quality Index, the 30-year record showed an improvement from Poor to Good. The index reached its lowest value, 42.4, corresponding to a Poor rating, in 1993–1994, and from 2000 onwards fluctuated between Fair and Good. Dieldrin, conductivity, sulfates, and lindane were the most influential variables in that calculation. Over the full period, water quality was never ranked as excellent, although the authors note that improvements in specific parameters, particularly the banned pesticides, raised the overall score.

The index proved sensitive to its inputs. When the researchers calculated a separate index for the recent 2018–2019 period using the newer MIRU monitoring program, which measured 181 parameters, the overall rating was only Marginal to Good, between 62.3 and 79.7. In that recent calculation, nutrients and phytoplankton indicators were associated with the lowest index values, indicating that eutrophication remains one of the main processes affecting water quality. The river’s average total phosphorus concentration of 0.06 milligrams per liter confirms its eutrophic status, and seasonal algal blooms, often originating in the Salto Grande reservoir and the Negro River reservoirs, are well documented along the main channel.

The authors emphasize that the study is a synthesis of historical public monitoring data, not new field sampling, and that it faced limitations including inconsistent measurement frequency, changing detection limits, changing parameters over time, and few biological variables in the older records. Only eight traditional pesticides were analyzed consistently across the full period, representing less than five percent of the 227 active ingredients registered in Uruguay as of December 2025. Recent monitoring has detected more than 40 pesticide residues in river water, at least 11 above chronic or acute toxicity limits for biota, alongside pharmaceuticals and microplastics in fish. The team argues that maintaining standardized long-term monitoring, integrating discharge measurements, and adapting to constantly emerging contaminants remain critical challenges for managing this large, trinational river system.

Subject of Research: Long-term water quality trends and the role of river discharge in the mid-lower Uruguay River from 1987 to 2019

Article Title: Historical changes and role of river discharge in water quality: 30 years of environmental monitoring in Uruguay River

Article References: Somma, A., Lucas, C., & González-Bergonzoni, I. (2026). Historical changes and role of river discharge in water quality: 30 years of environmental monitoring in Uruguay River. Ecological Processes, 15(1), Article 95. https://doi.org/10.1186/s13717-026-00753-x

Image Credits: AI Generated

DOI: 10.1186/s13717-026-00753-x

Keywords: Uruguay River, water quality, long-term monitoring, CCME-WQI, pesticides, eutrophication, river discharge, metals, nutrients, Salto Grande reservoir, transboundary basins, Bayesian modeling